add a serial test lane for concurrency-vulnerable tests

Generalizes the pty split into an explicit "parallel=false" opt-out: a
`serial` tag (declared in dart_test.yaml). The parallel flutter run now
excludes `pty || serial`; a separate `flutter test --tags serial
--concurrency=1` pass runs the vulnerable ones. For the coverage gate the
two passes are real-merged by ci/merge_lcov.py (union DA, max hits, recompute
LF/LH) — a plain concat would double-count and corrupt the total.

Tag transcript_publisher's bus-republish test serial (it flaked in the
parallel pool). Gate verified green end-to-end at 95.08%.

T-193.

Co-Authored-By: Claude <noreply@anthropic.com>

ci/merge_lcov.py

@@ -0,0 +1,49 @@
+#!/usr/bin/env python3
+"""Merge lcov files into one, on stdout.
+
+clide runs coverage in two passes — the parallel pool and a serial
+(`--tags serial --concurrency=1`) pass for tests that can't share the
+parallel runner (T-193). Each `flutter test --coverage` pass overwrites
+coverage/lcov.info, and the coverage gate naively sums LF:/LH: across
+records, so a source file appearing in BOTH passes would double-count.
+
+This unions DA (line→hits) per source file, taking the MAX hit count (a
+line executed in EITHER pass counts as hit), then recomputes LF/LH. Line
+coverage only — which is all flutter emits and all the gate reads. No
+`lcov` dependency.
+
+Usage: merge_lcov.py a.info b.info [...] > merged.info
+"""
+import sys
+
+files = {}   # source path -> {line: hits}
+order = []   # first-seen order, for stable output
+
+for path in sys.argv[1:]:
+    cur = None
+    with open(path) as fh:
+        for raw in fh:
+            line = raw.rstrip("\n")
+            if line.startswith("SF:"):
+                cur = line[3:]
+                if cur not in files:
+                    files[cur] = {}
+                    order.append(cur)
+            elif line.startswith("DA:") and cur is not None:
+                num, _, hits = line[3:].partition(",")
+                num, hits = int(num), int(hits)
+                files[cur][num] = max(files[cur].get(num, 0), hits)
+            elif line == "end_of_record":
+                cur = None
+
+out = []
+for sf in order:
+    da = files[sf]
+    out.append("SF:" + sf)
+    for num in sorted(da):
+        out.append(f"DA:{num},{da[num]}")
+    out.append(f"LF:{len(da)}")
+    out.append(f"LH:{sum(1 for h in da.values() if h > 0)}")
+    out.append("end_of_record")
+
+sys.stdout.write("\n".join(out) + "\n")

ci/test.sh

@@ -30,10 +30,22 @@ echo "==> dart test (pty — unreliable under the flutter test runner; serial)"
 # for resource-bound tests, vs. the old per-test `retry:` band-aid. (T-193)
 dart test --concurrency=1 --tags pty test/pty/session_test.dart test/panes/registry_test.dart
 
+# The parallel pool excludes both pty (runs under dart test, above) and
+# serial-tagged tests (concurrency-vulnerable — run in their own --concurrency=1
+# pass below). See dart_test.yaml + T-193.
 if [[ "$coverage" == 1 ]]; then
-  echo "==> flutter test --coverage (gate; unit + widget + golden + a11y)"
-  flutter test --coverage --exclude-tags pty --timeout 60s
+  echo "==> flutter test --coverage (parallel pool; excludes pty + serial)"
+  flutter test --coverage --exclude-tags "pty || serial" --timeout 60s
+  cp coverage/lcov.info coverage/lcov.parallel.info
+  echo "==> flutter test --coverage (serial-tagged; --concurrency=1)"
+  flutter test --coverage --tags serial --concurrency=1 --timeout 60s
+  echo "==> merge coverage (parallel + serial passes → coverage/lcov.info)"
+  python3 ci/merge_lcov.py coverage/lcov.parallel.info coverage/lcov.info > coverage/lcov.merged.info
+  mv coverage/lcov.merged.info coverage/lcov.info
+  rm -f coverage/lcov.parallel.info
 else
-  echo "==> flutter test (dev; no coverage, parallel)"
-  flutter test --exclude-tags pty --concurrency=12 --timeout 60s
+  echo "==> flutter test (dev; parallel pool, excludes pty + serial)"
+  flutter test --exclude-tags "pty || serial" --concurrency=12 --timeout 60s
+  echo "==> flutter test (dev; serial-tagged, --concurrency=1)"
+  flutter test --tags serial --concurrency=1 --timeout 60s
 fi